Dear Dr. Universe: Why does it hurt when we pull a muscle?

Our big, strong, stretchy muscles can help us run, jump and play. But sometimes these muscles stretch a little too far, and it can really hurt.

Before we get into the part about pain, it helps to know what makes up a muscle: Fibers. You can imagine these fibers kind of like a handful of uncooked spaghetti noodles.

Of course, muscle fibers are much thinner than noodles. You can have millions of fibers in a muscle. That’s what I found out from my friend Bert Tanner, a researcher at Washington State University who knows a lot about both engineering and muscle biology.

You also have muscle tissue, Tanner said. You can think of the tissue kind of like play-dough that holds together all of the fibers, or noodles. Altogether, you have about 600 different muscles in your body.

It turns out these fibers are also what we call muscle cells. Cells are like building blocks. The whole human body is made up of lots of different cells, including hair cells, eye cells and skin cells.

When you pull a muscle, you are actually tearing some of the muscle cells. That’s what I found out from David Lin, a researcher at WSU who is curious about engineering and biology, too.

Lin told me that when the cell tears, the cell signals some pain sensors in your body to start firing away. These nociceptors send a message to your spinal cord and the brain. They help you know that you are in pain and that you should avoid any more damage.

Even though these muscle cells can rip, tear and pull, they also do a pretty good job at repairing themselves, Lin said. And while too much stretch may cause a muscle strain, a healthy amount of stretching can sometimes do the body good.

When we stretch our muscles, it can help clear our mind and let us focus on our body. I also like to stretch out my muscles when I wake up from a nap. Maybe you do this when you wake up, too.

Humans can stretch out their muscles during yoga or after playing sports. When they do this, it can release some chemicals called endorphins to the brain, which can make the person feel happy. You could even take a quick stretch break right now.

Everyone’s muscles are a little different, so you want to do the stretches that are best for you. Maybe you’ll reach up to the sky, make your arms wide or touch your toes.

While humans have some pretty strong muscles, some animals in our world have super strong muscles. If you were a kangaroo rat, your muscles could help jump 10 times your own hip height. You also could jump and land on really rocky surfaces without pulling a muscle. Lin is studying the behaviors of these animals to inspire new ideas for design and engineering. It turns out we can learn a lot from nature and even our own muscles, when we ask big questions.

Dr. Universe

Do you have a question? Ask Dr. Universe. Send an email to Washington State University’s resident scientist and writer at Dr.Universe@wsu.edu or visit her website, askdruniverse.com.